Discussion
The nuclei in the cells you have observed are undergoing mitosis. The chromosomes are not
separately visible in the nucleus of a cell which is not dividing. When the cell is about to divide, the
chromosomes first appear as a tangled mass of fine threads in the nucleus. Cells in onion or plant root
tips are usually dividing by mitosis.
Some chromosomes may appear as if they are split lengthways into identical halves called
chromatids. You may have noticed some are chromosomes arranged in a more or less regular manner.
Some chromatids may appear to be separate from their partners and at opposite directions of the
nucleus. You may also notice two sets of chromosomes with membranes around them.
You may notice that the arrangement of the chromosomes in different cells corresponds to the
phases in mitosis. Chromosomes will appear thick, short and replicated in prophase. In metaphase,
they are aligned along the equator. In anaphase, the chromatids separate and move away from each
other to opposite ends. In telophase, the chromatids have reached the poles of the cell as
chromosomes. The nuclear membranes have reformed and the cell divides into two.
Importance of mitosis
Mitosis is important because it is responsible for the following:
The process of asexual reproduction. This is due to the formation of new cells which retain the same
number and exact copies of chromosomes as the parent cell.
Mitosis is also the cause of growth when new cells formed increase in number and contribute to the
growth of an organism.
It is also important because when cells are damaged or die in the body, they are replaced when
existing cells divide by mitosis to provide additional cells.
Most animal cells are capable of dividing by mitosis when the need arises. Plant cells however are
not all capable of dividing by mitosis. Only a specialised group of plant cells called meristem cells
are able to do so. In single celled organisms like amoeba and other actively dividing cells, the
process of mitosis is cyclic. This means that when the daughter cells formed mature, they too undergo
mitosis. In specialised tissue like nerve tissue, division of cells stops once the cells mature.
This is why certain injuries to the spinal cord and the brain which is made up of nerve cells
sometimes leave the patient paralysed permanently. This is because some of the nerves when severed,
cannot be replaced. The existing nerve cells are too specialised to undergo mitosis to form
replacement nerve tissue.
Each daughter nucleus ends up with a set of chromosomes, which are identical. After the nucleus
divides, the cytoplasm then divides. This results in two cells that can exist independently.
A simple way of understanding mitosis is by first following what generally happens to the
chromosomes in the nucleus before following it up with what is also going on in the entire cell.
Normally, chromosomes only become clearly visible during cell division. The following is an
outline of the main events that involve only the chromosomes in an imaginary cell with two
chromosomes. Note that we will ignore the structure of the rest of the nucleus and the cell contents.
(i) Each chromosome duplicates itself by dividing into two strands called chromatids before the cell
divides. The chromatids lie side by side and are joined together at a point called a centromere.